In the case of transmitting a digital signal through a transmission line, part of a signal energy is reflected at the receiving side at a timing when a signal level changes, which leads to a problem that a waveform distortion such as overshoot or undershoot, that is, ringing occurs. Up to now, various techniques for suppressing the waveform distortion have been proposed. For example, as shown in FIG. 22, Patent Literature 1 discloses a technique for suppressing ringing by impedance matching only for a certain period during which a communication is not affected when a voltage level of the transmission line transitions between low and high levels.
In a CAN (Controller Area Network, registered trademark) which is one type of vehicle communications, differential signals of the low level and the high level, that is, data bits are referred to as recessive and dominant, respectively. In this case, as shown in FIG. 23, it is assumed that a ringing waveform changes with large amplitude in order of (1) dominant, (2) recessive, (3) dominant, and (4) recessive.
Then, in a configuration of Patent Literature 1, ringing suppression operation is started in response to a first change from (1) to (2), and when a fixed time has passed, the suppression operation is stopped in the next (3). Then, the ringing suppression operation is resumed in response to a next change to (4). For that reason, a time from the start of the first suppression operation in (2) to the stop after (4) is prolonged. Therefore, when it is assumed that a next bit, that is, the dominant is transmitted during the execution of the suppression operation, a current is drawn and a signal waveform is distorted.